CLHS: Issue FIXNUM-NON-PORTABLE Writeup

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## Issue FIXNUM-NON-PORTABLE Writeup

    Status: Passed Jan 89 X3J13, as amended
    Issue: FIXNUM-NON-PORTABLE
    References:	CLtL p. 14, 34, 43, 231
    Category:	CHANGE, CLARIFICATION

    Edit History:   Version 1, 11-Jul-88, Sandra Loosemore
    		    Version 2, 15-Sep-88, Masinter
    		    Version 3, 23-Sep-88, Masinter
    		    Version 4,  7-Dec-88, Masinter (two proposals)
    		    Version 5, 16-Mar-89, Masinter (incorporate amendments)
    		    Version 6, 17-Mar-89, Masinter (incorporate amendments correctly)

    Problem Description:

    Implementations of Common Lisp are required to support two disjoint
    subsets of integers, fixnums and bignums, with the promise that
    fixnums have a more efficient representation.  However, nothing is
    guaranteed about the range of integers which are fixnums: "Exactly
    which integers are fixnums is implementation-dependent; typically they
    will be those integers in the range -2**n to 2**n - 1, inclusive, for
    some n not less than 15."

    There are few uses of the fixnum type that are portable, given the
    current definition.  In particular, many programmers use FIXNUM type
    declarations where they really mean "small integer".

    While most Common Lisp implementations have a FIXNUM range
    which is a subset of integers represeted and operated on most
    efficiently, many also have several other subranges.  The
    partitioning of INTEGER into BIGNUM and FIXNUM is merely
    confusing in these implementations, and not useful.

    CLtL p14 and p34 disagree about BIGNUM. One says that
     FIXNUM and BIGNUM are an exhaustive partition of the
    integer space, the other says they might not be!

    Proposal: FIXNUM-NONPORTABLE:TIGHTEN-DEFINITION

    (1) Change the description of the type FIXNUM to reflect that it is
     required to be a supertype of (SIGNED-BYTE 16).

    (2) Define BIGNUM to be exactly (AND INTEGER (NOT FIXNUM))

    (3) require that (<= ARRAY-DIMENSION-LIMIT MOST-POSITIVE-FIXNUM)

    Example:

    Consider an implementation with three numeric representations:

    	Fast                (INTEGER -1024 1023)
    	Immediate           29 bits
    	Extended            Multi-precision

    Such an implementation would have to define
    FIXNUM to be (OR Fast Immediate). BIGNUM
    would then refer to multi-precision integers.

    Rationale:

    Many programmers already use FIXNUM to mean "small integer"; this
    proposal makes this usage portable.

    However, there is little portable use for the type BIGNUM, and it
    is inconsistent with many current implementation techniques.
    Removing it is an incompatible change for a weak reason.

    Current Practice:

    Xerox Common Lisp has 17-bit fixnums.  Most other Common Lisp
     implementations have  fixnum ranges of 24 bits or larger. We know
    of no implementation that currently violates the proposed minimum
    size.

    Several existing Common Lisp implementations have more than two
    representations for integers, such that the FIXNUM/BIGNUM distinction
    is confusing; they define BIGNUM to cover all of the larger number
    types.

    Cost to implementors:

    Slight.  All implementations we know of already define FIXNUMs to be at
    least 16 bits.

    Cost to users:

    Slight.

    Benefits:

    The FIXNUM type specifier would have a portable interpretation.

    The language would be less confusing.

    Discussion:

    There was little consensus on whether to leave BIGNUM in the language.

    Earlier discussion of a related proposal contained several other more
    controversial components (adding a constant MAX-INTEGER-LENGTH, allowing
    MOST-POSITIVE-FIXNUM to be NIL as well as an integer.) This proposal
    is an attempt to address the part that cleanup committee seemed to agree
    on.

    It is possible that an implementation have a single  representation for all
    integers, and no way to identify any efficient range of integers. Those
    implementations might need to set MOST-POSITIVE-FIXNUM
     and MOST-NEGATIVE-FIXNUM to arbitrary values, consistent with
    the requirement that (SIGNED-BYTE 16) is a subtype of FIXNUM.

    Other alternatives considered (and not necessarily mutually exclusive
    with this proposal):

      remove the FIXNUM type specifier entirely, while leaving a way
      to query what is the most efficient range of integers

       leave the range of FIXNUMs unconstrained  and introduce a
       SMALL-INTEGER type with a fixed range (but no promises about
       efficiency) .

    It might be possible to specify the required performance behavior
    of FIXNUMs more concretely, e.g., specify that the basic integer operations
    use algorithms that are not proportional to the size of the data;  it
    should be just about as fast to add numbers in the middle of the fixnum
    range as it is to add, say, 10 and 11. This might be a useful way to
    describe
    the intent of the FIXNUM range, if not its specification.

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